Macroevolution can be defined simply as evolution above the species
level, and its subject matter includes the origins and fates of major novelties
such as tetrapod limbs and insect wings, the waxing and waning of multi-species
lineages over long time-scales, and the impact of continental drift and
other physical processes on the evolutionary process. With its unique time
perspective, paleontology has a central role to play in this area: the
fossil record provides a direct, empirical window onto large-scale evolutionary
patterns, and thus is invaluable both as a document of macroevolutionary
phenomena, and as a natural laboratory for the framing and testing of macroevolutionary
hypotheses. This is a vibrant field (if underpopulated relative to the
wealth of material and questions within its domain), with a steady stream
of papers, books and symposia and an increasing interaction with a broad
range of disciplines from astrophysics to developmental biology. The result
has been a number of insights into the processes that have shaped the major
evolutionary patterns of present-day and ancient organisms.

Origins

One striking macroevolutionary pattern that has emerged from the fossil
record is that major groups and evolutionary novelties have not originated
randomly in time and space. The Cambrian Explosion at the beginning of
the Paleozoic Era established virtually all of the major body plans seen
in present-day oceans, along with a number of extinct, enigmatic groups,
a remarkable evolutionary burst reflected in the sudden appearance in the
fossil record of all but one of the living marine phyla and many of the
marine classes. Equally intriguing is the realization that both morphological
divergence and the production of higher taxa are less prolific after this
early Paleozoic pulse, although similar evolutionary pulses have also been
documented in vascular plants with the invasion of land, and in terrestrial
insects soon thereafter. One major question has been the relative role
of development (e.g. changes in canalization) and ecology (e.g. ecospace
availability) in promoting and then damping these evolutionary explosions.

In marine and terrestrial systems alike, secondary bursts of evolutionary
novelty appear to be concentrated in the wake of mass extinctions, when
taxonomic and morphologic divergence is again accentuated (although not
so dramatically as in the early Paleozoic). The timing of these originations
lends support to a macroevolutionary hypothesis of incumbency -- which
early occupants of a resource or habitat can dominate until an extinction
or other perturbation clears the way for other groups. Still unclear is
whether such mechanisms can explain spatial patterns in the origin of novelties,
such as the preferential onshore origination of higher taxa in Paleozoic
and post-Paleozoic marine invertebrates (and some possible analogs in Paleozoic
and post-Paleozoic terrestrial plants), and the preferential tropical origins
of many marine invertebrate orders and some terrestrial plant groups.

The mechanisms underlying the origins of novelties remain poorly understood.
Interactions with two biological fields have been especially valuable in
exploring this macroevolutionary problem, and are sure to accelerate in
the future. Phylogenetic analysis, including molecular techniques, can
give a genealogical context for the production of novel features, providing
essential data on the morphology, ecology and other aspects of probable
ancestors or less-derived outgroups; in exchange, paleontology provides
data on the age, distribution, and characters of extinct taxa that can
significantly change the interpretation of character state polarities and
the sequence of evolutionary transitions. Developmental biology can provide
insights at several levels. Developmental patterns at the organismic level,
for example, can help to explain how relatively small changes in the timing
of development, or heterochrony, can have dramatic morphological results.
At the molecular level, knowledge of the operation and phylogenetic distribution
of regulatory gene systems can elucidate how those genetic control systems,
some of them extremely ancient, were altered or commandeered to generate
novel morphologies. Selected references: Bengtson 1994, Benton 1987, 1996,
DiMichele & Aronson 1992, Donoghue et al. 1989, Erwin et al. 1987,
1997, Foote 1996, Jablonski 1993, Jablonski & Bottjer 1990a,b,c, Knoll
et al. 1984, Lipps & Signor 1992, McKinney & McNamara 1991, McNamara
1995, Padian et al. 1994, Smith 1994, Wagner 1995a.

Dynamics

Much macroevolutionary research was triggered by the realization that
many species appeared to be almost static morphologically after their first
appearance in the fossil record rather than evolving continuously. This
led to the hypothesis of punctuated equilibrium, which holds that most
evolutionary change accrues at the branchpoints of species' histories rather
than over the duration of established species. The fossil record does indeed
exhibit many examples of stasis, but contains not only instances of punctuated
equilbria, but also punctuated anagenesis (evolutionary histories consisting
of stasis and rapid change without branching), gradual anagenesis (constant,
directional evolution without branching) and even the gradualistic splitting
of lineages that characterized evolution in classical textbooks. Still
unclear is whether these different evolutionary tempos and modes are distributed
unevenly among taxa, habitats, regions, or ecological categories.

These observations have prompted an approach to certain macroevolutionary
patterns, such as trends in the morphology or species-richness within lineages
that allowed for the operation of evolutionary processes simultaneously
at several hierarchical levels. Thus, directional changes or shifts in
the number of species contained within subclades of larger groups might
be attributed to sorting among units at several different levels, from
the familiar one of bodies within populations to species or higher taxa
within clades. Factors that accelerate speciation rates within one segment
of a clade, for example, might shape the long-term development of that
clade as a whole. Differences in speciation or extinction rates have long
been recognized among and within higher taxa (e.g. ammonites vs. bivalves,
lytoceratid vs. acanthoceratid ammonites), but much more work is needed
on the biotic factors that might confer these rate differentials. Further,
separating species selection in the strict sense from the waxing or waning
of groups as a byproduct of processes at the individual level has proven
difficult in most instances. More generally, empirical research and quantitative
theory have lagged far behind the qualitative theoretical arguments in
this area.

As in the origin of novelties, phylogenetic information can be extremely
valuable in analyzing macroevolutionary dynamics: many macroevolutionary
hypotheses can be stated in terms of predictions about phylogeny, for example
on the repeated association of particular characters with changes in evolutionary
rate or the topology of an evolutionary tree, or on the variation in branch
lengths in different regions of an evolutionary tree. Phylogenetic methods
that take into account both the strengths and weaknesses of paleontological
data are being developed and debated; this interaction with systematic
biology will continue to be important, not only for paleobiological research,
but as a basis for testing hypotheses about molecular evolution, including
constancy or variation in rates among groups and through time (which can
be assessed directly for groups with accurate phylogenies and a rich fossil
record).

The relative roles of physical and biotic factors in shaping macroevolutionary
patterns also remain hotly debated, and surely vary among taxa, times,
and places. On the biotic side, rate differentials among higher taxa imply
a role for such inherent biological factors as genetic population structure
or intrinsic growth rates; the Mesozoic Marine Revolution and other apparent
escalations in predation intensity and in the defenses of potential prey
suggest that diffuse interactions among many disparate taxa can have long-term
consequences; and the effects of incumbency discussed above argue for at
least a pre-emptive mode of competitive interaction over macroevolutionary
timescales. On the physical side, changes in climate, geography, and perhaps
atmospheric or ocean chemistry have proven to have overriding effects in
some cases, and many of the biotic replacements once thought to reflect
competitive interactions now appear to have been mediated by mass extinction
(e.g. the replacement of dinosaurs by mammals at the end of the Mesozoic
Era). Still problematic is the issue of spatial and temporal scale: the
theoretical expectations from ecological theories of competition and predator-prey
interactions are difficult to extrapolate meaningfully to macroevolutionary
scales, so that new expectations tuned to the dynamics of clades over millions
of years need to be developed.

The role of extinction, particularly mass extinction events, has been
a major research area. Whether extinction peaks merely accelerate processes
set in motion during times of less intense turnover or play a more special
role in the evolutionary process is an area of active research, and the
role of extinction events may vary with their magnitude and timing. Some
have argued that mass extinctions appear to break the dominance of incumbent
groups and open opportunities for diversification and evolutionary novelties,
which may help to explain how such events can have such dramatic evolutionary
effects despite accounting for only a small proportion of the total extinction
that has occurred over the past half-billion years. More work is needed
on selectivity during mass extinctions, particularly comparative approaches
among extinction events of different intensities. Such studies can also
be important for understanding, and quite possibly predicting, extinction
and recovery patterns in the present-day biota as human-caused environmental
purturbations increase in extent and severity.

The history of multicellular life has been characterized by stable,
if weakly bounded, ecological/evolutionary associations at several scales,
from communities and community types to the three evolutionary faunas of
Phanerozoic oceans. The marine evolutionary faunas have intriguing parallels
on land, in the four evolutionary floras among vascular plants and the
three evolutionary faunas among tetrapod vertebrates. In each instance,
the major faunas/floras appear to have their own characteristic dominant
taxa, levels of diversity (increasing in stepwise fashion), and, at least
in the marine faunas, characteristic rates of origination and extinction
for those dominant taxa (decreasing in turn among the evolutionary faunas
--which may help to explain the coincident long-term evolutionary behaviors
of their constituent taxa). Still uncertain is exactly what maintains the
apparent environmental separation among marine faunas for much of the Phanerozoic,
whether similar environmental (or latitudinal) separation occurs in the
terrestrial equivalents, and the extent of mixing and dilution among the
faunas or floras over their histories. The problem of biotic interaction
and ecologic scale emerges again when considering the characteristic diversity
levels: why should marine diversity at the family level, for example, hold
steady for most of the Paleozoic despite extensive turnover (one possible
implication is the attainment of a global equilibrium, reset or perhaps
never even reached in post-Paleozoic times), and to what extent are the
successive higher characteristic diversity levels attributable to physical
versus biological controls? Selected references: Clyde & Fisher 1997,
Erwin & Anstey 1995, Gould & Eldredge 1993, Grantham 1995, Hitchin
& Benton,1997, Jablonski 1986, 1995, Jablonski & Sepkoski 1996,
McShea 1994, Norrell & Novacek 1992, Patzkowsky 1995, Sepkoski 1992,
1996, 1997, Valentine 1990, Vermeij 1987, 1994, Wagner 1996.

Methodologies

Macroevolutionary study has benefited from a steady infusion of quantitative
methods from many other fields, from bootstrapping and time-series statistics
to the scaling up of survivorship and rarefaction analysis, as well as
the generation of novel approaches to morphometrics, phylogenetics and
sampling theory. These quantitative tools will greatly expand the scope
and depth of macroevolutionary questions addressed in the fossil record,
as will the growing array of computerized databases -- which as noted above
should begin to incorporate a broader spectrum of information, such as
localities and environments (see databases report).

Mathematical models have also been used to good effect. The most extensively
explored methods to date have been models of taxonomic diversification,
in which the dynamics of clades or entire faunas are modeled with simple
equations to explore the effects of variations in extinction, origination,
and the strengths of interaction among and within groups. New work on Morphological
diversification and its relative concordance or discordance with taxonomic
patterns shows much promise. Models of individual body forms, such as Raup's
classic work on molluscan shells, have enjoyed less attention lately, but
the ability to generate a theoretical morphospace based on a few generative
parameters, and then to explore the occupation of that morphospace by living
and fossil organisms, remains a macroevolutionary avenue with great potential.
In all of these modeling approaches, the aim has not been simply to reduce
complex macroevolutionary patterns to a few simple explanatory equations,
but to improve understanding of how specific changes in each variable in
turn would shape macroevolutionary phenomena, and to develop a baseline
of expectations for random behaviors against which the patterns of the
fossil record can be compared (such as long-term trends in morphology or
taxonomic diversity).

The ultimate source of virtually all these data are the rocks and sediments
of the fossil record, and the quantitative assessment of potential biases
represents an important, expanding area for macroevolution. Taphonomic
studies, from actualistic assessment of the fidelity of local assemblages
to the quantification of the megabiases of available outcrop area and stratigraphic
gap analysis, have begun to provide taxon-, environment- and time-specific
assessments of the nature of paleontological data--and for the most part,
the news is good on the reliability of paleontological data. Methods for
placing statistical confidence limits on such variables as stratigraphic
ranges, taxon durations, diversity levels, and extinction intensities are
all being developed or refined, although the multitude of sampling situations
presented by the stratigraphic record requires much further work before
general methods are fully available. Even now, these approaches allow the
investigator to steer a more rigorous course between the defeatist view,
that the fossil record is too incomplete and biased to yield robust conclusions,
and the literalist one, that the fossil record should be taken strictly
at face value whatever its theoretical shortcomings or biases. Selected
references: Benton & Hitchin 1996, Donovan & Paul in press, Foote
1996a,b, Gilinsky & Signor 1991, Kidwell & Flessa 1995, Marshall
1997, Wagner 1995b.

Prospects And Needs

Macroevolution is an interdisciplinary field, and so one important goal
is to strengthen interchange with other branches of paleontology -- systematics,
biostratigraphy and paleoecology, for example can be crucial to developing
rigorous datasets for maroevolutionary study. Interactions with other areas
of geology, such as stratigraphy and geochemistry are also important, providing
a temporal framework and invaluable information on the environmental context
-- and perhaps even the forcing factors - of macroevolutionary patterns.
Electronic databases will play an important role, but targets should expand
beyond the synoptic taxonomic databases that have dominated the field,
to include morphological, ecological, phylogenetic and biogeographic information;
collecting and standardizing such data will be a major challenge in many
instances.

Interchange with the biological sciences is probably at an all-time
high; this has been beneficial in both directions, and should be encouraged.
Phylogenetics and developmental biology have already been mentioned as
rewarding avenues of interaction. The potential for interaction with ecology
is great, but much care is needed to consider the strengths and limitations
of the fossil record in terms of temporal and spatial scale. Ecologists
and microevolutionists are beginning to appreciate the importance of events
over larger time scales than the decades to centuries that are their usual
bounds, and a new effort is needed from both neontological and paleontological
sides to get beyond a simple extrapolation of ecological phenomena into
macroevolutionary time scales (see paleoecology report, presumably).

Besides an ongoing, healthy exchange with cognate fields in geology
and biology, more integration is needed within macroevolution itself. Taxonomic/phylogenetic
approaches and morphological approaches need to be more fully linked, although
this is beginning. Each approach has its strengths and may be better-suited
to a particular set of macroevolutionary problems, but neither is "closer"
to the evolutionary process, because understanding evolutionary process
at any hierarchical level requires knowledge of genealogy and the sorting
dynamics on the one hand, and of the phenotypes that interact with the
physical and biotic environment on the other. Cross-comparisons among different
approaches to reconstructing evolutionary history, for example morphology-
or molecule-based phylogenies versus stratigraphic distributions, will
be also continue to grow in importance, and should help to produce a more
rigorous approach to historical analysis in which methods are fitted to
the strengths and weaknesses of the system under study. Finally, as might
be expected in a young science (at least in its present incarnation), empirical
analyses have tended to lag behind theory. With a few exceptions, relatively
few detailed studies are available for any given problem; this is at least
partly a function of the labor-intensive nature of macroevolutionary research,
which necessarily must encompass significant periods of time, numerous
taxa and/or many characters. As hypotheses are framed in more specific
and rigorous terms, and as methods for assessing and even factoring out
sampling and other biases become increasingly available, this imbalance
should be remedied, and a rich balance of data and theory will continue
to inform research on large-scale patterns of evolution.

This page is maintained for the Paleo21 Organizing Committee
by Norman MacLeod and H.
Richard Lane. Corrections, inquiries about, and updates to any of the
information shown above should be directed to Norm and/or Rich.